Found in the Cathedral of the Deep. The player first needs to join Rosaria's Fingers and offer at least one Pale Tongue either to rank up in the covenant, reallocate attributes, or change appearance. The player must then talk to Leonhard who will appear at the entrance of Rosaria's Bed Chamber.
Used to invade Leonhard in the room above the Aldrich, Devourer of Gods bonfire (Gwynevere's chamber in Dark Souls). Upon defeating him in his world, the player will obtain the Soul of Rosaria, the Crescent Moon Sword and his Silver Mask.